This invention relates to laser gyroscopes employing waves of two or more different frequencies within a laser gyroscope cavity. More particularly, the invention relates to cancelling the Fresnel-Fizeau drag effect resulting from the phenomenon of Langmuir flow which otherwise causes gyro output bias drift.
In general, laser gyroscopes devices have two or more waves traveling in opposite directions along a closed path including a laser gain medium so that rotation of the device about an axis within the path causes the path length for oppositely rotating waves to differ depending upon the rate of rotation. With a two wave or frequency system, it has been found that, for low rates of rotation corresponding to a small theoretical difference frequency, the actual output difference frequency is zero or substantially less than would be expected due to the phenomena known as lock-in. It is believed that the lock-in problem arises because of coupling between the waves which may arise from a number of possible factors including back scattering of laser energy from elements within the laser path such as reflectors or a polarization dispersive structure or from scattering centers within the laser gain medium itself.
One of the most significant laser gyroscopes yet proposed and constructed employs four waves of two pairs each propagating in opposite directions. Such systems are shown and described in U.S. Pat. Nos. 3,741,657 and 3,854,819 to Keimpe Andringa and assigned to the present assignee, the specifications of those patents being herein incorporated by reference. In such laser systems, circular polarization for each of the four waves is used. The pair of waves, or beams, propagating in the clockwise direction includes both lefthand circularly polarized (LCP) waves and right-hand circularly polarized (RCP) waves as does that propagating in the counterclockwise direction. This four-frequency or multi-oscillator ring laser gyro provides a means of circumventing the lock-in problem present in all conventional or two-frequency laser gyroscopes. This approach may be described as two independent laser gyros operating in a single stable resonator cavity, sharing a common optical path, but statically biased in opposite senses by the same passive bias element. In the differential output of these two gyros, the bias then cancels, while any rotation-generated signals add, thereby avoiding the usual problems due to drifts in the bias and giving a sensitivity twice that of a single two-frequency gyro. Because the bias need not be dithered, the gyro never passes through lock-in. Hence, there are no dither-induced errors to limit instrument performance. For this reason, the four-frequency gyro is intrinsically a low noise instrument, and it is well suited for applications requiring rapid position update or high resolution.
The speed of light propagating in a moving medium depends on the velocity of the moving medium. In a laser gyroscope, a moving medium will drag the resonant light frequencies or laser beam waves along with the medium producing a frequency shift effectively simulating a rotation rate. This frequency shift is the Fresnel-Fizeau drag effect resulting in a gyro output bias.
A helium-neon gas discharge within a laser gyroscope is such a moving medium. The phenomenon of Langmuir flow, in which the heavy ions in the plasma are more strongly coupled to the walls of the gas-discharge tube than are the electrons, results in a net flow of gas down the center of the tube toward the cathode and a return flow along the walls in the opposite direction. Thus, there is a large gradient of velocity within a laser gyroscope cavity bore.
The Fresnel-Fizeau drag effect has been one of the earliest recognized error sources affecting two-frequency as well as multi-frequency laser gyroscopes. One prior art approach has been attempted to suppress or cancel said drag effect by a perfectly symmetric split discharge approach whereby a precise electronic current source (supplied to two anodes) is required to maintain equality of electric current flowing in each half of the split discharge path, but in opposite directions. The traveling resonant light frequencies encounter the gas flows set up by the split discharge currents and the drag effect of one discharge current tends to cancel the drag effect of the other discharge current.
Another approach in the prior art has provided for the generation of low frequency amplitude modulated currents to each of two anodes of a two-frequency ring laser gyro to produce a modulation in the speed of the gas discharge flow that results in a cancellation of the Fizeau effect. This approach, however, requires considerable electronic circuitry external to the optical ring laser cavity.
This invention causes the cancellation of the Fresnel-Fizeau drag effects on the resonant light frequencies without the need for two anodes and associated precision electronics external to the ring laser cavity.